In recent years, in a disk brake for a vehicle, such as, e.g., a four-wheel vehicle or a two-wheel vehicle, a brake piston of aluminum alloy is employed to attain high-performance and low-fuel consumption. The cup-shaped brake piston 1 shown in FIG. 1 is one example thereof (see Patent Document No. 1).
FIG. 2A shows a disk brake 10 in a brake-off state, and FIG. 2B shows a disk brake in a brake-on state. In the disk brake 10, when a liquid pressure is applied to an inside of a cylinder of a caliper 11 in which the brake piston 1 is accommodated, the open end portion of the brake piston 1 presses a back plate 13 of a friction pad 12, causing a contact of the friction pad 12 with the brake disk 14, which in turn results in a braking action.
The aforementioned cup-shaped brake piston can be produced by, for example, extruding a billet, drawing the extruded member into a bar-shaped member, subjecting the bar-shaped drawn member to solution treatment, subjecting the solution treated member to aging treatment to attain desired mechanical strength, and then machining the aged member into a cup-shape. Alternatively, it can be produced by, for example, casting a bar-shaped member of aluminum alloy, cutting the cast member into a predetermined thickness, forging the cut member into a cup-shape, subjecting the cup-shaped forged member to T6 treatment, and then machining the T6 treated forged member to improve the dimensional accuracy and remove distortion caused during the solution treatment (see Patent Document No. 2).
The brake piston is a sliding member. Therefore, the brake piston is required to have material strength and wear resistance capable of preventing deformation during the brake operation, and also is required to have machinability during the production and smoothness of the sliding surface to attain smooth braking when in use.
For the material of such a brake piston, high-strength and high-wear and abrasion resistance aluminum alloy in which elements, such as, e.g., Cu, Mn, and/or Cr, are added to an Al—Si series alloy or an Al—Mg—Si series alloy is used. In order to improve the wear resistance and the corrosion resistance, an anodic oxide film is formed on the alloy (see Patent Documents Nos. 2-4). As an aluminum alloy suitable for anodization, an Al—Fe series alloy used as an architectural material or a cap material for a beverage bottle is known (Patent Documents Nos. 5-7).
Patent Documents 2 and 3 describe that strength of an anodic oxide film is influenced by the existence state of Si particles.